Self-servo-writing multi-slot timing pattern

ABSTRACT

Self-servo-writing of multi-slot timing patterns is described. Individual timing marks are replaced with groups of timing mark slots. At each timing mark location, a time measurement is made by detecting a timing mark in one of the slots. Also, extensions to the existing timing marks are written in other slots. The combination of timing measurements at every timing mark and extensions to those timing marks written at every opportunity improves the overall precision of the timing propagation. The improved accuracy of timing mark placement produces a commensurate improvement in the placement of the concomitantly written servo-data. In addition, the alignment accuracy of the written pattern is less sensitive to variations in rotation speed and variations in the shape of written transitions. Moreover, only a single disk revolution is required at each servo radius to write servo data and propagate the timing marks to maintain timing alignment.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of the commonly-owned, U.S. patentapplication Ser. No. 10/184,343 entitled “Self-Servo-Writing Multi-SlotTiming Pattern,” filed on Jun. 27, 2002 now U.S. Pat. No. 7,019,926, theentire disclosure of which is herein incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of timing pattern generationfor self-servo-writing magnetic drives.

2. Description of Related Art

High track densities in rotating media mass storage devices are becomingpossible with newer drive technologies. These new technologies includevoice-coil and other types of servo positioners as well as the abilityto read and write narrower tracks by using, for example,magneto-resistive (MR) head technology. Higher track densities increasethe accuracy requirements of servowriting methods for embedded servosystems needed to position the head.

Conventional disk drive manufacturing techniques, for example, includewriting servo-tracks on the media of a head disk assembly (HDA) with aspecialized servo-writer instrument. Laser positioning feedback is usedin such instruments to read the actual physical position of a recordinghead used to write the servo-tracks. Unfortunately, it is becoming moreand more difficult for such servo-writers to invade the internalenvironment of a HDA for servo-writing because the HDAs themselvesdepend on their covers and castings being in place for proper operation.Also, some HDAs are very small, less than 2 inches square At such levelsof microminiaturization, traditional servo-writing methods areinadequate.

Conventional servo-patterns typically comprise short bursts of aconstant frequency signal, very precisely located offset on either sidefrom a data track's center line. The bursts are written in a sectorheader area, and can be used to find the center line of a track. Stayingon center is required during both reading and writing. Since there canbe 100 or more sectors per track, that same number of servo data areasmust be dispersed around a data track.

Further, the servo-data is generally dispersed around the data track bywriting short bursts in each of the hundred or so sector header areas ofthe data track. Such data bursts can be used by the embedded servomechanism to find the center line of the data track. This allows thehead to follow the track center line around the disk even when the trackis out of round (e.g., due to spindle wobble, disk slip, and/or thermalexpansion). As the capacity of disk drives increases track density islikewise increased, the servo-data must be more accurately located onthe disk.

Servo-data is conventionally written by dedicated, externalservo-writing equipment, and typically involves the use of graniteblocks to support the disk drive and quiet outside vibration effects. Anauxiliary clock head is inserted onto the surface of the recording diskand is used to write a reference timing pattern. An external head/armpositioner with a very accurate lead screw and a laser displacementmeasurement device for positional feedback is used to preciselydetermine transducer location. This precise transducer location is thebasis for track placement and track-to-track spacing. The servo writerrequires a clean room environment, as the disk and heads are exposed tothe environment to allow the access of the external head and actuator.

A conventional servo-data pattern on a disk comprises circular datatracks that are broken into sectors. Each sector typically has a sectorheader area followed by a data area. Each sector header area includessector header information followed by a servo-data area that providesradial position information. The sector header information includes aservo-identification (SID) field and a gray code field that must beprecisely aligned from track to track to prevent destructiveinterference in the magnetic pattern. Such interference can reduce theamplitude of the signal and cause data errors.

During conventional drive manufacturing, the disk drive is typicallymounted in a mastering station that is known as a servo-writer. Theservo-writer has sensors that are positioned outside of the disk driveto locate the radial and circumferential position of at least one of thedrive's internal heads. Using information from the sensors, theservo-writer causes the head to write a pattern, typically magneticinformation, (i.e., servo-data) onto the disk. As explained above, theservo-pattern becomes the master reference used by the disk drive duringnormal operation to locate the tracks and sectors for data storage. Whensuch a station is used to perform the servo-writing, manufacturingexpenses increase because each disk drive must be mounted in theservo-writer. Additionally, the mechanical boundary conditions of thedisk are altered because the external sensors must have access to theactuator and the disk spindle motor. Thus, mechanical clamping anddisassembly of the drive may also be required.

According to another conventional servo-writing process, a master clocktrack is first written on the disk by a separate head to serve as atiming reference for the entire servo-track writing operation. Afterwriting the master clock track, servo-data bursts are written over theentire surface of the disk by first moving the arm to the outer crashstop and then radially moving the arm a distance that is less than adata track width using an external radial positioning system for eachrevolution of the disk.

Such conventional servo-writing procedures require the use of anexternal timing sensor in order to write the timing patterns that areused to determine the circumferential head position. Because externalsensors are needed, the servo-writing must be performed in a clean roomenvironment. Additionally, an external clock source and auxiliary clockheads are required to write the timing information.

To overcome such problems, self-servo-writing timing generationprocesses have recently been developed. These processes allow accuratelyaligned servo-data tracks to be written sequentially at each servo dataradius without using any mechanical, magnetic, or optical positioningsystems to control the circumferential positioning of the servo data.Further, the need for auxiliary clock heads to write a reference timingpattern on the disk is eliminated.

According to one method, first timing marks are written at a firstradial position of the storage medium. Time intervals between selectedpairs of the first timing marks are measured. The head is moved to asecond radial position. Next, additional timing marks are written byrecording the time of passage of every other timing mark (say the oddnumbered ones) during revolutions of the disk and then writing theintervening time marks (the even numbered ones) at calculated delaysthereafter. The time intervals between the newly written (even) marksare estimated to be the difference in times of passage of the adjacenttiming (odd) marks plus the difference in the delay before writing thenew timing marks. Then the head is moved to a second radial position.Next, additional timing marks are written by recording the time ofpassage of every other timing marks at the circumferential positionsjust written (here the even numbered ones) during revolutions of thedisk and then writing the intervening time marks (the odd numbered ones)at calculated delays thereafter.

The time intervals between the newly written (odd) marks are estimatedto be the difference in times of passage of the adjacent timing (even)marks plus the difference in the delay before writing the new timingmarks. In the preferred method, servo data is written on one or moredisk surfaces in the intervals between the timing marks. In a preferredmethod, the steps of measuring, moving, and writing other timing marksare repeated until the servo-pattern is written on an entire surface ofthe storage medium.

While such self-servo-writing processes are sufficient when theservo-data tracks are to be written using overlapping read and writeheads (i.e., where a track can be written and read without changing headposition), disk drives with non-overlapping read and write elements arenow being produced.

More specifically, as read and write element dimensions have beendecreased to increase storage density, the widths over which reading andwriting occur have decreased more rapidly than the distance between theread and write elements themselves. As a result, when using a head withsuch elements on a rotary actuator, the read element of the head can nolonger overlap the area written by the write element of the head at allradial positions. When the above self-servo-writing processes are usedfor drives in which the read and write elements do not overlap, accuratecircumferential alignment of the servo-data tracks is not maintained andthere is a lack of stability against the growth of random errors in thepattern generation process.

According to another method, first timing marks are written at a firstradial position of the storage medium during revolutions of the disk.Then the head is moved to a second radial position. Time intervalsbetween selected pairs of the first timing marks are measured duringrevolutions of the disk. Next, additional timing marks are written byrecording the time of passage of every other timing mark (say the oddnumbered ones) during revolutions of the disk and then writing theintervening time marks (the even numbered ones) at calculated delaysthereafter. Then the head is moved to a second radial position. Timeintervals between selected pairs of the first timing marks are measuredduring revolutions of the disk. Next, additional timing marks arewritten by recording the time of passage of every other timing marks atthe circumferential positions just written (here the even numbered ones)during revolutions of the disk and then writing the intervening timemarks (the odd numbered ones) at calculated delays thereafter. In apreferred method, the steps of moving, measuring, and writing othertiming marks are repeated until the servo-pattern is written on anentire surface of the storage medium.

Commonly owned U.S. patent application Ser. No. 09/592,740, filed Jun.13, 2000 and entitled “Method for Self-Servo Writing Timing Propagation”is hereby incorporated by reference in its entirety. This U.S. patentapplication (heretofore referred to as the '740 application) describes aself-servo-writing process. The placement of new timing marks hadpreviously occurred at least every other revolution to allow reading of,and measuring all the time intervals between, existing timing marks atleast during a revolution before writing a subsequent new timing mark.In addition, with all of these processes only half of the timing marklocations are written at each radial position. This, unfortunately, canresult in odd-even sector asymmetry, reduced signal strength at thetiming mark, and increases the overall time between measurements duringwhich the motor speed can significantly vary possibly introducingadditional timing errors into measurements of timing mark locations.

Further, another type of reading and writing apparatus uses an offsethead. In an “offset” head, the read and write elements are physicallyseparated in the radial direction. An offset head includes a recordinghead, otherwise known as a write head, and a magnetic detection head,otherwise known as a read head. A prior art offset writing processrequires that additional timing measurements be made to maintain processstability which adds to process time.

The invention of the '740 application overcame problems with the priorart by detecting both the passage of the timing marks and writingextensions to timing marks at substantially the same circumferentialpositions. This is feasible even if a disk drive or similar system isunable to write and read at the same time, if the read head is aseparate element that encounters points on a disk surface slightlybefore the write head as the disk rotates so that the detection of anexisting timing mark can take place before the writing of a timing markin the same circumferential location. After the read operation occursthis delay allows the subsequent write operation at substantially thesame tangential location on the same revolution. Using this processwhich records the passage of every timing mark and then writesextensions to all of those timing marks at substantially the samecircumferential positions improves the accuracy of timing mark placementand produces a commensurate improvement in the placement of theconcomitantly written servo data.

Another invention allows this last higher accuracy method where alltiming mark locations are both detected and written during the same diskrevolution to be executed without performing any other measurement stepsexcept those made during that disk revolution. This reduces the overallprocess time. This invention is disclosed in co-pending U.S. patentapplication Ser. No. 09/426,435, filed Oct. 25, 1999 and entitled“Storage of Timing Information for Self-Servo Writing Timing PatternGeneration When Read and Write Heads are Non-Overlapping”, which definesa location array which stores the estimated intervals between newlywritten timing marks calculated from the measured timing mark intervalsof the timing marks that are detected by the read head and the delaysused to write new timing marks in a data array. This U.S. patentapplication (heretofore referred to as the '435 application) is commonlyassigned herewith to International Business Machines, and is herebyincorporated by reference in its entirety. At each new writing step thestored estimated interval data for the timing marks currently passingunder the read head is retrieved and used to predict the correct delaysfor writing. This means it is not necessary to measure the timeintervals between the timing marks passing under the read head beforethe disk revolution that new timing marks are written.

The combination of the inventions of the '740 application and the '435application allows for both 1) high accuracy, since according to theinvention of the '740 application every timing mark is both read andwritten on each step in the same revolution (rotation) of a disk in adrive, and 2) high process speed, since according to the invention ofthe '435 application it is not necessary to take the time to measure thetime intervals between the timing marks passing under the read headbefore the disk revolution that new timing marks are written. Howeverthe invention of the '740 application requires particularly, heads thatcan read and then write at the same circumferential location on the samerevolution via the presence of a significant delay that allows thesubsequent write operation at substantially the same tangential locationon the same revolution. The delay requirement also limits the duration(circumferential extent) of a timing mark that can be written since thereading of the timing mark must be completed before the write elementreaches the leading edge of the set of extensions to that timing mark.This constraint limits the types of recording head and timing markpatterns that can be used with the higher accuracy method where everytiming mark is both read and written in the same revolution.

Accordingly, there exists a need to overcome the problems with the priorart as discussed above, and particularly for a method to moreeffectively write timing marks on rotatable storage media.

SUMMARY OF THE INVENTION

Briefly, in accordance with the present invention, disclosed is a systemand method for improved self-servo-writing of multi-slot timingpatterns. Individual timing marks are replaced with groups of timingmark slots. At each timing mark location, a time measurement is made bydetecting a timing mark in one of the slots. Timing marks are written inother slots. The combination of timing measurements at every group andtiming marks written at every group improves the overall precision ofthe timing propagation. The improved accuracy of timing mark placementproduces a commensurate improvement in the placement of theconcomitantly written servo-data. In addition, the alignment accuracy ofthe written pattern is less sensitive to variations in rotation speedand variations in the shape of written transitions. Moreover, only asingle disk revolution is required at each servo radius to write servodata and propagate the timing marks to maintain timing alignment.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the present invention will become moreapparent from the detailed description set fourth below when taken inconjunction with the drawings in which like reference numbers indicateidentical or functionally similar elements.

FIG. 1A is a diagram showing the arrangement of a timing mark and arecording transducer, which includes a write head and a read head,according to one embodiment of the present invention.

FIG. 1B shows a disk drive showing the arrangement of an actuator usedto position a recording transducer on recording media, according to oneembodiment of the present invention.

FIG. 2 is a grid showing the arrangement of generated timing marks,according to one embodiment of the present invention.

FIG. 3 is a flowchart depicting the overall operation and control flowof the timing mark generation process, in one embodiment of the presentinvention.

FIG. 4A and FIG. 4B are flowcharts depicting a detailed description ofone step in the operation and control flow of the timing mark generationprocess, in two different embodiments of the present invention.

FIG. 5. is a table of formulae for the write timing slots, read timingslots following incrementing, and write delays for different read slotcases, in one embodiment of the present invention.

FIG. 6. is a table of formulae for the write timing slots, read timingslots following incrementing, write delays and the correction parameterC[S,i] for different read slot cases, in one embodiment of the presentinvention.

FIG. 7 is a diagram of the sector layout of timing mark groups on arotating media, in one embodiment of the present invention.

FIG. 8 is a block diagram of an exemplary self-servowriter timing systemuseful for implementing the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is described in terms of the exemplary embodimentsbelow. This is for convenience only and is not intended to limit theapplication of the present invention. In fact, after reading thefollowing description, it will be apparent to one of ordinary skill inthe relevant art(s) how to implement the present invention inalternative embodiments.

Actuator Geometry

FIG. 1B shows a disk drive showing the arrangement of an actuator 150used to position a recording transducer 152 on recording media 148,according to one embodiment of the present invention. The actuator 150positions the recording transducer 152 to write tracks on the recordingmedia 148 at any radial location. The actuator positional informationmay be derived from various methods well know in the art, either byself-servowrite radial propagation or by external sensors.

FIG. 1A is a diagram showing the arrangement of a set of timing markextensions 160 and a recording transducer 152, which includes a writehead 154 and a read head 156, according to one embodiment of the presentinvention. The direction of the media rotation 148 is counterclockwisewhich is shown as right to left in FIG. 1A for set of timing markextensions 160. The timing marks are written sequentially and thedirection of timing mark propagation is vertical (lower to upper). Disktracks are shown as oriented horizontally in FIG. 1A. FIG. 1A shows theposition of the recording transducer 152, which includes a write head154 and a read head 156 relative to a set of timing mark extensions 160at a fixed radial location.

Timing marks 162 thru 170 are written sequentially. Timing mark 162 iswritten first, followed by the writing of timing marks 164, 166,168 andfinally 170. Each sequentially written timing mark overlaps thepreviously written timing mark. As shown in FIG. 1A, timing mark 170overlaps 168, 168 overlaps 166, 164 overlaps 162. As can be shown inFIG. 1A, the write head 154 does not overlap the same timing mark as theread head 156. That is, a timing mark cannot be written by the writehead 154 and read by the read head 156 without changing the radiallocation of the actuator 152.

FIG. 1A shows that while read head 156 is positioned over the track areacorresponding to timing mark 162, the write head 154 is positioned overthe track area corresponding to timing marks 168 and 170. As explainedabove, chronologically, timing mark 162 was written first, timing mark164 was written second, timing mark 166 was written third, timing mark168 was written fourth and timing mark 170 was written fifth—the mostrecent.

Timing mark 164 is a timing mark extension of 162. A timing markextension has been defined previously in the '740 application to be atiming mark written at least in part at the same circumferentiallocation as, and coherently and aligned with, the data of some portionof an existing timing mark at a neighboring radial location such thatsome part of the two timing marks can be read simultaneously by the readhead 156 at some radial position. The goal is to maintain a precisealignment of the timing marks so as to provide exact indicators of therotational position of the disk during the servo-write process. Also, aradial trajectory is the area on a rotatable storage medium, such as adisk, that is defined by the area spanned by an initial timing mark andthe set of timing mark extensions to that initial timing mark. Anapproximation to the final written radial trajectory may bepredetermined as a target trajectory along which the timing marks are tobe written.

Thus, at a given radial position, read head 156 can read timing marks162 and 164, which were written three and four timing steps back,respectively. This feature is a result of the width of the read head156, the width of the write head 154, the radial separation 174 betweenthe read head 156 and the write head 154, called the head offset, andthe radial length of the timing marks from 162-170. The radial length ofthe timing marks 162-170 is set by the step size of the radial stepbetween timing mark extensions 172. In another embodiment of the presentinvention, the distance between the read head 156 and the write head 154is such that the read head 156 can read timing marks which were writtena predetermined number of timing steps back. The parameter N1 as shownin FIG. 1A is defined as the integer part of (largest integer less than)the ratio of the read to write radial head offset 174 minus half thewrite head width 154 over the step size S, given by:N1=Integer part of (Offset−Write/2)/SFurther, N2 is defined as N1+1.

In one embodiment of the present invention, each track of a disk isdivided into V sectors, wherein V is a whole number multiple of theproduct sector count as shown in FIG. 7 where V=8. Within each sector,there are M possible timing slots wherein M is a whole number-greaterthan two. In one example, as shown in FIG. 7, M=3. In the preferredembodiment of the present invention, timing marks are grouped togetherwith a fixed time interval t between each timing mark. The time intervalt between the timing marks is determined by the velocity of therecording media upon which the timing marks are recorded and thedistance between the timing marks d and is given by: t=v/d, where v isthe velocity of the recording media.

The total time interval between the first and second timing marklocations must be of sufficient time so that after a timing mark iswritten in the first timing mark location, the read head 156 and readback electronics in the disk drive have substantially recovered from thedisturbance caused by writing prior to the read head passing overbeginning of the second timing mark. A similar duration is maintainedbetween the penultimate and last timing mark locations. In the preferredembodiment, M is 3 and the physical location of the timing slots aredefined so that slot 1 722 begins some fixed time as the disk rotatesafter the disk index 700 in FIG. 7. Slot 2 724 begins a fixed time, t,after the beginning of slot 1 722 such that t is equal to or greaterthan the sum of the time required to write a timing mark and the maximumtime required by the read and write heads and electronics tosubstantially return to their steady state condition. Slot 3 726 beginsthe time t after the start of slot 2 724. In a preferred embodiment allwritten timing marks for a group are written separated by times whichare the same for all timing groups, but the times may be chosen to bedifferent.

Timing Mark Arrangement

FIG. 2 is a grid showing the arrangement of generated timing marks,according to one embodiment of the present invention. FIG. 2 shows thedetailed arrangement of timing marks for sector 0 720 and sector 1 730of FIG. 7. The direction of disk rotation in FIG. 2 is horizontal (rightto left) while the direction of timing mark propagation is vertical(lower to upper). Grid 205 shows the locations on a disk, onto whichtiming marks are written within each sector. The vertical columns (0-1,0-2, 0-3) represent the timing slots available for timing marks, whilethe rows corresponds to tracks of the recording media. Two sectors areshown in FIG. 2, each containing three timing slots in what is referredto as a timing mark group. The first timing mark group, group 0 labeled0-1,0-2, 0-3, is shown in the first three timing slots (columns) of thegrid. The second timing mark group, group 1, labeled 1-1, 1-2, 1-3, isshown in the second three timing slots (columns) of the grid. At theleft of the grid are lines indicating the radial locations of the readhead, labeled 209 to 289. The filled and shaded boxes indicate timingmarks. For example, when the read head is located at the radial position239 the write head is at a radial position to write timing marks at 232,231, 238, and 237. For the purpose of FIG. 2, N1 as described above ischosen to be two (N1=2) as an example. In the preferred embodiment, theoffset is a slowly varying function of the actuator position and can beany value greater than or equal to two radial steps.

The present invention describes how to extend a start-up pattern oftiming marks (crosshatched in FIG. 2) 200, 201 and 206, 207 so as tomaintain time alignment. It is assumed for this discussion that thestart-up timing marks 200, 201 and 206, 207 have already been writtenvia prior art. The description of the invention focuses on the writingof timing mark group 1. It is also assumed that all other timing markgroups are written in the same manner as timing mark group 1, asdescribed herein. In the preferred embodiment of the present invention,three timing slots are shown for each timing mark group. In anotherembodiment of the present invention, any whole number of timing slotsgreater than 2 are used for each timing mark group. When a timing markis written one radial step later and at the same timing mark slot as apreviously written timing mark, a radial portion of the previouslywritten mark is overwritten (erased and replaced) by the new timingmark. For example, the timing mark 211 has been partially overwritten bythe timing mark 221. The timing mark 261 has no immediate successor intiming slot 1, so it is not overwritten.

As a introduction to the detailed discussion of FIG. 2 to follow, therules governing the sequence of writing and reading can be summarized.In the preferred embodiment, with three timing slots in each group, thetime of passage of one timing mark is recorded (read) and two timingmarks are written at the same radial position. Servodata is writtenbetween the timing mark groups using the time of passage of the readtiming marks to maintain servopattern timing alignment as shown in FIG.7.

The sequence of reading and writing the timing patterns in the timingslots is described. The process begins with reading one timing mark andwriting the other two timing marks for a single revolution of the disk.At the next radial step, the sequence proceeds as follows:

1. If the read timing slot has been repeated for a number of radialsteps less than N1, keep the read timing slot the same.

2. Otherwise change the read timing slot, in the preferred embodiment,increment the read timing slot by one or, if the current read timingslot is the last one (e.g. slot 3 in FIG. 1), then set it to the firstread timing slot.

In FIG. 2, timing marks shaded with diagonal lines are those that arewritten in timing slots directly following the read timing slot, whilethe uniformly shaded timing marks are written in the remaining timingslot.

In an embodiment of the present invention, with reference to grid 205 ofFIG. 2, the timing pattern generation process begins with thepositioning of the actuator 152 such that the read head 156 ispositioned at 219.

As explained above, the geometry of the recording transducer results inthe write head 154 being positioned over 211 and 217. Timing marks arewritten in pairs. The time of passage of timing mark 200 is recorded andthe first timing mark is written at 211, in the second timing slot. Thenext timing mark is written at 212, in the third timing slot. As thedisk continues to rotate, the time of passage of the timing mark in thefirst timing mark slot (206 for group 1) is recorded and additionaltiming marks are written in slots 2 and 3 (217 and 218 for group 1) foreach successive timing mark group. Writing in slots 2 and 3 and readingin slot 1, continues until the last timing mark group in sector V-1 haspassed under the recording transducer.

Next, the recording transducer 152 is positioned such that the read head156 is positioned at 229. Again the timing mark 200 is read (the term“read” is used as a shorthand for observing the read back signal andrecording the time of passage of the timing mark in a timing slot.) inslot 1, and the next pair of timing marks are written at 221 and 222.Writing and reading continues in successive timing mark groups as thedisk rotates until the last timing mark groups V-1 have passed under therecording head. In the remainder of this discussion, writing and readingcontinues in successive timing mark groups as the disk rotates until thelast timing mark groups V-1 have passed under the recording head will beassumed whenever writing of timing marks is described.

At this juncture, the recording transducer 152 has moved N 2 steps whilereading in the first timing slot. For the case described by FIG. 2,where the read to write head offset is such that N1=2, two is the numberof timing steps before the reading is moved to the next timing slot.Having moved two steps, the read timing slot is incremented, in thiscase to slot 2. This is a result of the radial separation between thewrite head 154 and the read head 156, as explained previously In theinvention, moving to the next timing slot after a number of steps equalto N1 has beneficial features because it creates timing marks thatprovide a reference which is dependent on more than one timing slot. Theorigin of these benefits will be described more fully below.

Next, the recording transducer 152 is positioned such that the read head156 is positioned over 239. The next pair of timing marks are written at231, in the third timing slot, and 232, in the first timing slot. Therecording transducer 152 is then positioned such that the read head 156is positioned over 249. The next pair of timing marks are written at 241and 242. At this juncture, the actuator 152 has moved N=2 steps in thesecond timing slot. As before, two is the maximum number of timing stepsallowed before the read slot 152 is required to move to the next timingslot. Since the previous read was in the second timing slot, the readtiming slot is moved to the third or last slot. Next, the recordingtransducer 152 is positioned such that the read head 156 is positionedover 259. Timing marks are written at 251 and 252. The recordingtransducer 152 is then positioned such that the read head 156 ispositioned over 269. The next pair of timing marks are written at 261and 262.

As before, the actuator 152 has moved N=2 steps, the maximum for thecurrent head offset, in the third timing slot. Thus, the read slot movesto the next timing slot, since the previous timing slot was the last,the read timing slot is moved to slot 1, the first. The recordingtransducer 152 is then positioned such that the read head 152 ispositioned over 279. The arrangement of read slots and write slots arethe same as at step 1, when the read head was positioned at 219. Thetiming mark in the first slot is read and the other timing slots in thegroup written in an analogous way. The process of writing timing marksand incrementing the read slot every time the number of steps moved isequal to N1 is repeated until the complete servo pattern has beenwritten across the disk.

One advantage of the present invention is that in each contiguoussection of timing marks in each timing slot (e.g., 212, 231, 241) thereare at least two radially adjacent timing marks that have been writtenwhile reading timing marks in different timing mark slots. The sequenceof writing and reading is defined so that at a subsequent radialposition the read head is in a position so that it spans portions ofboth of these timing marks while additional timing marks are written. Inthe preferred embodiment, this event occurs at the last step before theread slot is incremented. An example is when the read head is positionedat 259 and reading occurs in slot 3 at the timing mark indicated by 250.At this position, the timing mark is made up of 222, written when theread slot was timing slot 1 and 231 written when the read timing slotwas slot 2. These features are beneficial because they provide areference which is dependent on more than one timing slot, which couplestiming information between all three timing slots. This prevents timingmark errors from propagating by linking the propagation in each slot totwo (or both in the preferred embodiment) of the other slots. In theabsence of this three-slot coupling, errors in the relative position oftiming marks in different timing slots can accumulate and degrade theaccuracy of the timing placement.

Alternative embodiments that preserve this coupling or linking ofmultiple timing slots are readily constructable. In one alternativeembodiment timing marks are written in only a single timing mark slotwhere the timing mark in the other write timing slot will never be read.In one alternative of this embodiment, timing marks 228, 237 and 247would be written but 218 would not. Both 227 and 228 would be written sothat the adjoining timing marks 228 and 237, which are written whilereading in different timing mark slots (0 and 1) respectively, wouldstill be present. In another embodiment, the order of the timing marksis rearranged. For example, slot 2 could be placed physically beforeslot 1, or after slot 3, or the slot order could be reversed. Oneskilled in the art can devise further arrangements trivially. Thepreferred embodiment described here uses the minimum number of timingmarks and is convenient because there are relatively few distinctprocess steps.

Timing Mark Generation Process

FIG. 3 is a flowchart depicting the overall operation and control flowof the timing mark generation process, in one embodiment of the presentinvention. Control flow begins with step 320 and flows directly to step322.

In step 322, the recording transducer 152 is moved to an initial radiallocation using actuator 150 where the read head 156 passes over thestart-up pattern of timing marks (200, 201, 206, 207 in FIG. 2.) and thewrite head 154 is positioned radially to write extensions to thestart-up timing pattern. In addition, the variable N, representing thenumber of timing steps in the current timing slot, is defined as zero.The read timing slot, RS is selected as slot 1. The first write timingslot WS1 is set to slot 2. The second write timing slot, WS2 is set toslot 3.

In step 323, the estimated intervals are calculated from interval datastored during the start-up pattern generation. The derivation of theestimated intervals, write delays and storage of measured intervals willbe described later. In the absence of start-up interval data, theintervals between timing marks in the read timing slot can be measuredduring a disk rotation. In step 324, the process holds during the diskrotation until just prior to the passage of the V-1 timing mark group.In step 325, the times of passage of timing marks in the read timingmark slots are recorded and timing marks are written in the other timingmark slots. The details of step 325 are explained further in thediscussion of FIG. 4 below, which describes two preferred embodiments ofthe invention.

In step 326, the recording transducer 152 is moved to a radial positionone servo step further from the starting point. The step numbers S and Nare incremented. In 327 the intervals between the times of passage ofthe timing marks in the read slots are stored, along with the parameterC[S,i], following the formulae described in the next section. In step328 the value of N is examined to determine how many radial steps havebeen made using the current timing slots. If N is equal to N1, asdescribed in FIG. 1, the process moves to step 329 where N is set tozero and then to 330, otherwise the process moves directly to step 330.In step 329 the read timing slot, RS, is incremented to the next latertiming slot, unless the read slot is the last (third) timing slot in thetiming mark group, in which case the read slot is set equal to the firsttiming slot—that is, RS=RS+1 unless RS=3, then RS is set to 0.

In step 330, the number of completed radial steps S is checked. If thisis equal to the number of steps in the servopattern, the processcompletes by exiting in step 331; otherwise the process returns to step323 to continue stepping radially and writing timing marks and theproduct servopattern. It should be noted that the product servopatternmay be written during the timing propagation process, but may also bewritten after the timing propagation is completed.

FIG. 4A and FIG. 4B are flowcharts depicting a detailed description ofone step (325) in the operation and control flow of the timing markgeneration process, in two different embodiments of the presentinvention. FIG. 4A and FIG. 4B describe the details of the process stepwherein the passage of timing marks in the read slots is recorded andnew timing marks are written. FIG. 4A illustrates process steps for afirst preferred embodiment which shall be referred to as Write WhileRead (WWR). FIG. 4B illustrates the process steps for a second preferredembodiment which shall be referred to as Direct Write While Read (DWWR).

Write While Read Process

The details of step 325 in FIG. 3 are described in more detail for theWWR process in FIG. 4A by breaking step 325 into sub process steps 441to 452. Step 325 enters the process of FIG. 4A with step 441. In theprevious step (324) in FIG. 3, the process has waited until just beforethe last timing mark group. Following step 441 is step 442 where thetiming mark group index, i, is set to the last timing mark group V-1 andthe process waits for the timing mark. The passage of the timing marktriggers the start of the hardware delay T0 with time W0.

In the next step 443, we calculate the delay times for the write delaysW1[i] and W2[i] for the first and second write timing slots respectivelyvia the formulae described in the next section and in FIG. 5. The flowdiagram is shown split in the next step between the flow controlfunction 446 and the simultaneously occurring sequence of hardwareprocesses 444 and 445. The control flow goes to 446 from 443 and theprocess waits for both the T0 delay to be started and for the T2 delayto elapse, which indicates the completion of the timing mark group.

During this wait the hardware timer T0 expires which starts the T1 andT2 timers 444. The T0 timer automatically resets to the delay W0 onexpiration. As the T1 and T2 timers expire 445 they trigger the writingof new timing marks in two of the timing marks slots. Also, the timingmark in the read slot is detected which starts the timer T0 again. WhenT0 has been started, indicating the timing mark has been read, and T2has expired, indicating two timing marks have been written, the processcontrol 446 moves on to step 450.

In step 450, the process checks to see if the last timing mark group haspassed—that is, whether i=V-1, indicating that timing marks have beenread or written at all timing mark slots around the circumference of thedisk in which case the process moves to step 452 and returns to FIG. 3;otherwise the process moves to step 451. In step 451, the timing markgroup index “i” is incremented. Following step 451 the process returnsto step 443 to continue in the next timing mark group reading a timingmark and writing two timing marks.

In this particular embodiment it should be noted that because the twowritten timing marks of each group are written with a spacing determinedwithout reference to the read timing mark of that group, they can beconsidered to be a single, long timing mark with blank space(s) in it.Obviously the composition of this mark changes with radial position(read and write slot number).

Direct Write While Read

The details of step 325 in FIG. 3 are described in more detail for theDWWR process in FIG. 4B by breaking step 325 into sub-process steps 461to 492. Step 425 enters FIG. 4B with step 461. In the previous step 324in FIG. 3, the process has waited until just before the last timing markgroup. Following step 461, is step 462 where the hardware triggers areconfigured to start each of the delays T1 and T2 on either the detectionof the timing mark or the elapsing of delay T0 depending on the readtiming mark slot as indicated in FIG. 6. The process waits in step 463until the read timing mark in the last timing mark group (V-1) haspassed, which triggers the start of delay T0 with delay W0.

In the next step 464, we calculate the delay times for the write delaysW1,W2 for the write timing delays T1 and T2 via the formulae describedin the next section and in FIG. 6. Control flow is shown split in thenext step to show the simultaneous control flow 465 and hardwareprocesses. The hardware steps vary depending on the read timing slotindicated by the RS value of 1, 2 or 3 as indicated in FIG. 4B. Afterstep 464, the process control section waits at step 465 for T0 to berunning and T2 to have expired.

In the case where the read timing slot is 3, the hardware is configuredso that when T0 expires 470, it starts delays T1 with delay time W1 anddelay T2 with delay time W2. The timer T0 automatically resets to thedelay W0 on expiration. When T1 expires 471, a timing mark is written inslot 1. Next, when timer T2 expires, a timing mark is written in timingmark slot 2. Finally in step 473, the timing mark in slot 3 is detected,starting timer T0 again.

If the read timing slot is slot 2, the hardware is configured so thatwhen T0 expires 480, it starts timer T1. The timer T0 automaticallyresets to the delay W0 on expiration. When T1 expires 481, a timing markis written in slot 1. Next in step 482, the timing mark in slot 2 isdetected, starting timer T0 again and timer T2. Next, when timer T2expires 483 it causes the write of a timing mark in timing mark slot 3.

If the read timing slot is slot 1, first the timing mark in slot 1 isdetected 490, starting timer T0, T1 and T2. T0 expired previously but isnot configured to start any delays. When T1 expires 491, a timing markis written in slot 2. Next, when timer T2 expires 492, it causes thewrite of a timing mark in timing mark slot 3.

Regardless of the read timing slot, each of hardware processes completesthe current timing mark group with T0 running and T2 expired. When thishappens, the control flow step 465 continues to step 466. In step 466,the process checks to see if the last timing mark group has passed—thatis, whether i=V-1, indicating that timing marks have been read orwritten at all timing mark slots around the circumference of the disk,in which case the process returns to FIG. 3 at step 467; otherwise theprocess moves to step 468. In step 468, the timing mark group index “i”is incremented. Following step 468, the process returns to step 464 tobegin reading and writing in the next timing mark group.

Delay Calculations

The present invention follows a technique disclosed in the '435application, which defines a location array that stores the location ofthe i-th timing mark group or an array of timing mark intervals. In thepreferred embodiments of the present invention, we define an array ofstored intervals AI[S,i]; S corresponds to the radial position (orradial step or track number) of recording transducer 152, while “i”represents the sector number within which a timing mark is written, suchthat:AI[S,0]=IM[0,V-1]−A+D[0]−D[V-1]AI[S,i]=IM[i,i−1]−A+D[i+1]−D[i]where IM [j, k] is the time of passage of the timing mark in read timingslot of the “j”th timing mark group minus the time of passage to thetiming mark in the read timing mark slot of the “k”th timing mark groupduring the disk revolution where the writing of new timing marks istaking place at the “S”th radial step. We will use the variable D[i] torepresent the sum of the systematic plus random_error terms as definedin the '740 application. We will use the variable A to represent anycorrections to the interval for variations in the rotation speed,following the methods of prior art. Finally we will describe how to usethose delays in the WWR and DWWR embodiments for writing for the case oftiming mark groups. Also, the index S is incremented after each radialstep. In the preferred embodiment, to reduce the size of the data array,modulo K arithmetic is used for the index S where K is a number at least1 larger than the maximum read to write radial head offset. While notshown explicitly, modulo arithmetic is implied for S everywhere. Also,modulo arithmetic is implied for the interval indices “i”.Derivation of Intervals and Write Delays

The '435 application also teaches the calculation of current estimatedintervals I[i+1, i] from stored information. The estimated intervals canbe used to calculate write delays which indicate how long after a timingmark the write should be executed to extend the existing timing marksoptimally. The “i” index is defined as before to indicate the “i”thtiming mark group.

In the preferred embodiments of the present invention the estimatedintervals are:I[i+1,i]=AI[S−N3,i+1]*F1+AI[S−(N3+1),i+1]*F2where the parameter N3 is the integer part of the current read to writeradial offset measured in servo steps, and F1 and F2 as defined in the'435 application. In a preferred embodiment, if the fractional part ofthe of the current read to write radial offset measured in servo stepsis less than 0.2, then set F1=0.2; if the fractional part of the of thecurrent read to write radial offset measured in servo steps is greaterthan 0.8, then set F1=0.8; otherwise F1 is the fractional part of thecurrent read to write radial offset measured in servo steps. In thissame embodiment F2=1−F1.

The '740 application teaches how to calculate delays between timing markdetections and writing of extensions to those timing marks for the caseof single timing marks rather than timing mark groups, using intervalsmeasured or estimated between single timing marks: delays between thedetectable timing marks and the desired writing locations for additionalmarks are calculated utilizing the estimated time between the previoustiming mark and the one being extended. The estimated times between theprevious timing mark and the one being calculated can be preferablycorrected for systematic delays according to the teachings of U.S.patent application Ser. No. 09/550,643 and U.S. patent application Ser.No. 08/882,396 (now U.S. Pat. No. 6,251,732 which are commonly owned bythe assignee of the present invention and are incorporated herein byreference in their entirety. Additionally, in a preferred embodiment,corrections for errors accumulated from previous process steps areintroduced according to the teachings of U.S. patent application Ser.No. 09/316,884, U.S. patent application Ser. No. 09/316,882, and U.S.patent application Ser. No. 08/891,122, which are all commonly owned bythe assignee of the present invention and are incorporated herein byreference in their entirety.

The '740 application teaches two embodiments with different delays forwriting. The first embodiment is equivalent to the embodiment of thecurrent invention called WWR. In this embodiment, the delay for writingis either the estimated or measured interval between the timing markfrom which a delay is measured and the timing mark at thecircumferential position to be written, plus the systematic and randomerror corrections:delay=interval+systematic+random_error.

The second embodiment is equivalent to the embodiment of the currentinvention called DWWR. In this embodiment, the delay for writing is thesum of systematic and random error corrections:delay=systematic+random_error.

The '740 application then teaches how to calculate delays for writingfrom (estimated) intervals. We will use the variable D[i] to representthe sum of the systematic plus random_error terms as defined in the '740application We will use the variable A to represent any corrections tothe interval for variations in the rotation speed, following the methodsof prior art. Finally we will describe how to use those delays in theWWR and DWWR embodiments for writing for the case of timing mark groups.

Calculations of Delays for Writing (WWR Embodiment).

FIG. 5 is a table of formulae for the write timing slots, read timingslots following incrementing, and write delays for different read slotcases, in one embodiment of the present invention. FIG. 5 summarizesparameters and formulae for the write delays W1[i] and W2[i] for thepreferred WWR embodiment for each of the read slot locations. Forexample, if the read time slot is slot 1, the delay W1 set for the T1delay timer when the group index is “i” is W1[i]=D[i]+I[i−1,i]+A−W0+dwhere D[i] is the sum of the interval+systematic+random_error as definedin the '740 application for the “i” timing mark. The estimated intervalsused to calculate the interval, systematic delay and random_error termshave been replaced by the estimated interval as defined above and in the'435 application, and d is the predetermined time spacing between timingmark slots as defined earlier, and W0 is the predetermined delay for thetimer T0.

Calculations of Delays for Writing (DWWR Embodiment)

FIG. 6 is a table of formulae for the write timing slots, read timingslots following incrementing, write delays and the correction parameterC[S,i] for different read slot cases, in one embodiment of the presentinvention. FIG. 6 summarizes parameters, hardware configuration andformulae for the write delays W1 and W2 for the preferred DWWRembodiment for each of the read slot locations. For example if the readtime slot is slot 1, the delay W1 set for the T1 timer when the groupindex is “i” is W1[i]=D[i]+d−CC[i] where D[i] is the sum of thesystematic+random_error as defined in the '740 application for the “i”timing mark. The measured or estimated intervals used to calculate theinterval, systematic delay and random_error terms have been replaced bythe estimated interval as defined in the '435 application using thedefinition of the stored intervals modified as described above. Theparameter CC[i] is defined below.

For another example, if the read slot is 2, the delay time W2 isD[i]+I[i−1,i]+A−d−CC[i−1] for the T2 delay timer for the “i”th timingmark group. FIG. 6 also indicates the hardware triggering setup of thetimers T1 and T2. These two timers can be configured to be automaticallystarted by either the detection of a timing mark or the elapsing ofdelay timer T0, with the configuration depending on the current readtiming slot as indicated in the table of FIG. 6. For instance, if theread timing slot is 2, the T1 delay is started by the elapsing of delaytimer T0 while the delay timer T2 is started by the detection of thetiming mark.

In the DWWR preferred embodiment of the present invention, we define acorrection term C[S, i], indexed by step number and timing group numberin a way analogous to the stored timing mark locations L[S,i]. Equationsfor C[S,i] are given in the next section. C[S,i] provides an estimatethe amount which the location of a timing mark must be corrected inorder to reflect the ideal position of that timing mark. When timingmarks are written based on the time of passage of a previous timing markgroup, rotation speed variations and position errors in the previousgroup can introduce errors in the placement of the new timing mark. C[S,i] stores the estimate of this error based on the time of passage of atiming mark in the current timing mark group. Initially, all of thevalues of C[S, i] are zero. Formulae for C[S,i] are given in the tableof FIG. 6 for the current read timing slot in terms of the measuredinterval IM[i,i−1] and the write delays W1[i] and W2[i] and number ofsteps, N, taken at the current read timing slot location and the currentread to write offset.

For example, if the read timing slot is 2 and the parameter N is equalto N1, then C[S,i] is IM[i,i−1]−W2[i]−d. If the read timing slot is 2and the parameter N is less than the integer part of the read to writeoffset N1, C[S,i] is zero as shown in the table of FIG. 6.

The current estimate of the correction term is defined, indexed only by“i”, CC[i]. In the preferred embodiment of the inventionCC[i]=F1*C[S−N3,i]+F2*C[S−(N3+1),i].The parameters F1 and F2 are weighting factors that are a function ofthe head offset. In the preferred embodiment, N3, F1 and F2 are the samefactors defined above. Thus, the misplacement of the detectable timingmark is assumed to be the weighted average of the stored estimates C[S−N3, i], C [S−(N3+1), i], with the weighting being determined by therelative fractions of the read head falling over the portions of thetiming mark written N3 and N3+1 steps back. In an alternativeembodiment, C [i]=sum j=0 to k {Fj*C [S−Nj, i]} where the Fj are a setof k weighting factors applied to a number of stored data sets Nj backin the indexing.

The current estimate of the correction factor CC[i] is used when theplacement of a new timing mark is determined by detecting the passage oftiming marks which were themselves written at a position determined by adelay following a timing mark in the previous timing mark group. Thecorrection shifts the time of writing of the new timing mark to accountfor the misplacement (CC[i]) of the timing mark that starts the writedelay timer.

Exemplary Implementations

While the invention is shown for a rotational recording media, theinvention can be implemented for any system in which the recording mediamoves along any arbitrary trajectory including but not limited to linearmotion.

The present invention can be realized in hardware, software, or acombination of hardware and software. A system according to a preferredembodiment of the present invention can be realized in a centralizedfashion in one computer system, or in a distributed fashion wheredifferent elements are spread across several interconnected computersystems. Any kind of computer system—or other apparatus adapted forcarrying out the methods described herein—is suited.

A preferred embodiment of the invention can be realized in a system 900combining a computer system and software with specialized electronichardware as illustrated in FIG. 8. In FIG. 8, a master controller 902,which may be a computer or signal processor governs the overall sequenceof operations and communicates over a communication bus 903 withsubsystem elements such as a radial position controller 904, motorcontroller 916 and timing processor 906. The radial controller 904 setsthe position of the actuator arm and may be one of several typesincluding mechanical positioners or radial self-servowrite positioningsystems. The motor controller spins the disk drive motor and provides amotor index to the timing interval analyzer. The timing processor 906manages the self-servowrite timing functions of the current invention.The processor controls the process sequence. Such a processor can haveattached memory 905. Time measurement functions can be performed by timeinterval analyzer electronic 907 which measures the time intervalsbetween trigger patterns detected by the trigger pattern detector 908and trigger patterns and the motor index from the motor controller.Power and control signals to, and readback signals from, the disk drivebeing written 901 pass through the read/write interface 909. The writecontrol signals for the timing mark trigger patterns are generated inthe trigger pattern generator 916. The trigger pattern generator causesthe writing of a new timing mark when started by one of the programmabledelays W1, 914 or W2, 915. Another programmable delay W0, 913 is used asdescribed in the text earlier to control the starting of the W1 and W2delays and the process sequence. The selection of timing trigger signalsfrom the time interval analyzer, and W0,W1 and W2 delays to start theother delays is controlled by the delay control logic 910. The writingof the actual servo pattern data is controlled by the servopatterngenerator, with the timing of the servo data placement controlled by theprogrammable delay 911.

Although specific embodiments of the invention have been disclosed,those having ordinary skill in the art will understand that changes canbe made to the specific embodiments without departing from the spiritand scope of the invention. The scope of the invention is not to berestricted, therefore, to the specific embodiments. Furthermore, it isintended that the appended claims cover any and all such applications,modifications, and embodiments within the scope of the presentinvention.

1. A method for propagating a plurality of timing marks for servo dataalignment on a storage medium, the method comprising: detecting apassage of at least a portion of a first timing mark from a set oftiming marks, wherein the first timing mark is located at a first radiusand a first circumferential location; waiting for a delay period toexpire and writing a second timing mark, wherein the second timing markis located at a second radius and a second circumferential location;moving a recording head to a second radial location; detecting a passageof at least a portion of a third timing mark from a set of timing marks,wherein the third timing mark is located at a third radius and whereinthe third timing mark is not a timing mark extension of the first timingmark; and waiting for a delay period to expire and writing a fourthtiming mark, wherein the fourth timing mark is located at a fourthradius and wherein the fourth timing mark is an extension of the secondtiming mark.
 2. The method of claim 1, further comprising: moving therecording head to a new radial location; detecting a timing mark that isnot an extension of one of the first timing mark and the second timingmark; and writing a timing mark that is an extension of the fourthtiming mark.
 3. A method for propagating a plurality of timing marks forservo data alignment on a rotatable storage medium, the methodcomprising: detecting a passage of at least a portion of a first timingmark from a set of timing marks, wherein the first timing mark islocated at a first radius and a first circumferential location; waitingfor a delay period to expire and writing a second timing mark, whereinthe second timing mark is located at a second radius and a secondcircumferential location; detecting a passage of at least a portion of athird timing mark from a set of timing marks; and storing a timedifference between the writing of the second timing mark and the passageof a third timing mark as a parameter C[S,i], where C is a correctionparameter for different read slot cases, S is a step number and i is atiming mark number.
 4. The method of claim 3, further comprising: movinga recording head radially; detecting a passage of at least a portion ofthe second timing mark from a set of timing marks; and waiting for asecond delay period to expire and writing a fourth timing mark, whereinthe fourth timing mark is located at a fourth radius.
 5. The method ofclaim 4, wherein the second delay depends at least in part on theparameter C[S,i].
 6. A method for propagating a plurality of timingmarks for servo data alignment on a rotatable storage medium, the methodcomprising: arranging a plurality of timing marks into timing groups,wherein the timing groups comprise timing mark slots; and writing timingmarks using a first timing mark as a trigger timing mark; wherein inresponse to when a radial movement proceeds an integral number of stepsclosest to a write head offset minus half the write head width, thentiming mark writing is stepped to a next timing mark slot.
 7. The methodof claim 6, wherein the writing timing marks further comprises: whenwriting within a timing group, using correction terms C [S,i] and C [i]to write timing marks while minimizing errors, where S is a step index,i is a timing mark index, and C [i] is a current estimate of acorrection term.
 8. The method of claim 7, wherein the errors beingminimized comprise rotational speed variations and positional errors. 9.The method of claim 7, wherein the current estimate of C [i] furthercomprises a weighted average of stored estimates.
 10. The method ofclaim 9, wherein the weighted average of stored estimates comprises C[i]=F1*C [S−N1, i]+F2*C [S−N2, i], where the weighted average is arelative fraction of a read head overlapping at least a portion of atiming mark written N1 and N2 steps back.
 11. The method of claim 9,wherein the weighted average of stored estimates comprises C [i]=sum j=1to k {Fj*C [S−Nj, i]}, where Fj are a set of k weighting factors appliedto a number of stored data sets Nj.
 12. A method for propagating aplurality of timing marks for servo data alignment on a storage medium,the method comprising: detecting a first timing mark with a read sensorat a first radial location; and writing after a delay a second timingmark, which is not an extension of a previously written timing mark,wherein a circumferential location of a write element of the read sensorat a time of detecting is such that a leading edge of at least one ofall possible sets of extensions of the first timing mark has alreadypassed under the write element.
 13. A method for propagating a pluralityof timing marks for servo data alignment on a storage medium, the methodcomprising: generating at least one set of timing marks, the setcomprising at least one timing mark and at least one timing markextension such that the set is of radial length sufficient to allowwriting of a timing mark extension to a timing mark of the set at afirst radial sensor location wherein a read element of a sensor candetect the at least one of the set; detecting a first timing mark withthe read element of the sensor at the first radial location; and writingafter a delay a second timing mark, which is not an extension of one ofthe set of timing mark extensions, wherein a circumferential location ofa write element of the sensor at a time of detecting of the first timingmark is such that a leading edge of any set of timing mark extensions tothe first timing mark has already passed under the write element of thesensor.
 14. The method of claim 13 wherein the set of at least onetiming mark extension comprises two timing mark extensions.
 15. A methodfor propagating a plurality of timing marks for servo data alignment ona storage medium, the method comprising: detecting at least one firsttiming mark with a read sensor at a first radial location; writing aftera first delay at least one second timing mark, which is not an extensionof a previously written set of timing mark extensions; and writing aftera second delay at least one third timing mark, which is an extension ofpreviously written timing mark.